Multiple beam cathode-ray tubes generate, scan, and focus a plurality of electron beams as a group. Cathode-ray tubes of this type are capable of displaying pixel image data of high brightness at relatively high pixel data rates.
Multiple beam cathode-ray tubes typically include a resistive linear helix coil that is wound on the inner surface of the tube. A potential difference applied to the two ends of the helix coil generates a linearly varying potential along the length of the coil. This potential accelerates the electron beams and converges them onto an image surface that is positioned upstream of a display screen. Focus coils and deflection coils generate magnetic fields that, respectively, focus the image surface on and scan the electron beams across the display screen.
Cathode-ray tubes employing helix coils for acceleration and convergence of the electron beams typically include a drift tube section that receives a relatively low potential. As a consequence, such cathode-ray tubes suffer from unacceptable beam-to-beam compression. For increasing amounts of electron beam current, beam-to-beam compression is observed on the display screen as a narrowing of the vertical distance separating adjacent horizontal lines formed by the scan of the electron beams. Moreover, helix coils are expensive to manufacture and accumulate electrical charge during the operation of the cathode-ray tube. Charge accumulation on a helix coil generates spurious electric fields, which degrade the convergence performance of the helix coil and, thereby, cause a suboptimal focusing of images on the display screen.